Add hook for registering providers before metadata collection

[Jameswlepage]

Problem

The API_Credentials_Manager::collect_providers() method collects provider metadata during AI_Client::init(). This happens early in the WordPress init action, making it difficult for plugins to register additional providers that appear on the credentials screen.

Currently, any providers registered after AI_Client::init() runs will:

• Be available in the runtime registry
• Not appear on the Settings → AI Credentials screen (because collection already happened)

Current Workaround

Plugins must carefully order their initialization to register providers before calling AI_Client::init():

// 1. Init HTTP discovery first
WP_AI_Client_Discovery_Strategy::init();

// 2. Register custom providers to the registry
AiClient::defaultRegistry()->registerProvider(MyProvider::class);

// 3. Then init AI_Client (which collects providers)
AI_Client::init();

This is fragile and requires knowledge of internal implementation details.

Proposed Solution

Add a hook before provider collection:

private function collect_providers(): void {
    global $wp_ai_client_providers_metadata;

    if ( ! isset( $wp_ai_client_providers_metadata ) ) {
        $wp_ai_client_providers_metadata = array();
    }

    // Allow plugins to register providers before collection
    do_action( 'wp_ai_client_before_collect_providers', AiClient::defaultRegistry() );

    $registry = AiClient::defaultRegistry();
    // ... rest of collection logic
}

This would allow plugins to simply:

add_action( 'wp_ai_client_before_collect_providers', function( $registry ) {
    $registry->registerProvider( MyCustomProvider::class );
});

Alternative

Collect providers lazily on admin_menu (only when the credentials screen is actually needed) instead of eagerly during init.

[jeffpaul]

@felixarntz @JasonTheAdams would be helpful to get this into an upcoming release (as it impacts folks extending the base set of providers, example WordPress/ai#148)